There are many applications for high energy direct current electrical pulses of short time duration. For example, a xenon flashlamp of the type used to pump a solid state laser may require a pulse of direct current electrical energy having a pulse width of about 500 microseconds with a total energy in excess of 20,000 joules. The ideal pulse would be an exponentially rising pulse followed by a fast decay with a peak power approaching 100 megawatts at starting voltages in excess of 30,000 volts and peak currents approaching 5,000 amperes. There are applications in which a number of xenon flashlamps are operated simultaneously in parallel with each other thus creating a demand for a source capable of providing more than a million joules of energy in a 500 microsecond pulse at the voltage required and with sufficient total peak current to operate all of the flashlamps.
The most widely used method and means for generating a high energy electrical pulse of direct current includes the steps of accumulating and storing electrical energy in a capacitor bank over a period of time and then discharging all of the accumulated energy in a single pulse of short time duration. Very high voltage pulses at very high currents may be obtained in this way. However, the size and cost of the capacitor banks, used to accumulate the energy, increase in direct proportion to an increase in the energy of the desired pulse. For example, a capacitor bank capable of storing more than a million joules of electrical energy to be discharged in a single high voltage, high current pulse would require more than 4,000 cubic feet of space per million joules stored at a cost of more than $200,000 per million joules stored.
Direct current electrical energy may also be stored chemically as in a bank of batteries. However, batteries tend to be more massive than capacitors and would be at least as expensive and require at least as much space. Furthermore, the inherent internal resistance of battery cells impose limitations on their discharge capabilities, making it impractical to obtain an electrical pulse of short time duration therefrom at the high voltage and high current required to provide a million joules and more of electrical energy.
In the prior art, attempts have been made to generate high energy direct current electrical pulses of short time duration by instantaneous relative movement of a magnetic field with respect to the conductor. For example, high direct electrical currents have been established in the primary winding of a transformer and then interrupted to collapse the magnetic field and induce a high energy pulse in the secondary winding of the transformer. However, this requires the opening of a switch under high current conditions resulting in arc over at the switch, preventing the generation of a substantially exponentially rising pulse in the secondary winding. Thus, the pulse width and pulse shape obtainable with such inductive devices have not been suitable for use in pulsed power applications without the use of expendable switching devices which are complicated and expensive and render this application impractical.
Attempts have also been made to generate suitable high energy pulses using a homopolar generator consisting of a conductive disc mounted for rotation about its center on a conductive shaft in a pulsed magnetic field having field lines extending at right angles to the major surfaces of the disc. Electrical energy is extracted between the periphery of the disc and the shaft by means of brushes. Slow pulses of direct currents in excess of several hundred thousand amperes can be obtained. However, the voltage which it is practical to generate in a homopolar device is very low, being of the order of 100 volts, and it is not possible to obtain pulses shorter than 200 milliseconds due to the inductance of the field coils. Thus, some sort of energy storing system such as a capacitor bank must be used with a homopolar device to raise the voltage high enough for use in pulsing flashlamps and to enable the generation of a fast rising pulse.